In recent years, many of light-emitting elements used for displays and the like have a structure in which a layer containing a light-emitting substance is sandwiched between a pair of electrodes. Such light-emitting elements emit light when excitons which are formed by recombination of electrons injected from one electrodes and holes injected from the other electrodes return to a ground state.
In the field of light-emitting elements, in order to obtain a light-emitting element having high light-emitting efficiency and high chromaticity or a light-emitting element in which optical quenching and the like can be prevented, various researches about substances which become materials for manufacturing such a light-emitting element have been carried out.
For example, Patent Document 1 (Japanese Patent Application Laid-Open No. 2001-131541) discloses a material for an organic EL element having high light-emitting efficiency and long lifetime.
Further, in a light-emitting element, current flows between electrodes by transportation of holes or electrons. In this case, a light-emitting substance which has received holes or electrons or the like, or, a light-emitting substance which has been oxidized or reduced or the like sometimes does not return to a neutral state and is changed into a substance having a different property. When such changes of the property of the light-emitting substance are accumulated, a characteristic of the light-emitting element may also be changed.
Therefore, development has been demanded on a light-emitting substance of which a property is difficult to be changed due to oxidation or reduction.